TRAVERSAL

This repository contains an implementation of a multiword expression (MWE) identification system based on tree-structured conditional random fields. This system participated in the PARSEME shared task on automatic identification of verbal multiword expressions, edition 1.1, and ranked 1st in the general cross-lingual ranking of the closed track systems.

TRAVERSAL divides the task of MWE identification into two subsequent sub-tasks:

• Dependency tree labeling (with two possible labels: MWE and not-MWE)
• MWE segmentation (determining the boundaries of MWEs)

For the former task, the system encodes the possible labelings of dependency trees as tree traversals so as to capture unary, binary, and ternary relations between nodes, their parents, and their siblings. Then it strives to find the globally optimal traversal for a given dependency tree based on the multiclass logistic regression model.

For MWE segmentation, the system relies on a rather rudimentary solution -- by default, all adjacent dependency nodes marked as MWEs of the same category are assumed to form a single MWE occurrence.

Installation

First you will need to download and install the Haskell Tool Stack. Then clone this repository into a local directory and use stack to install the tool by running:

stack install

The above command builds the traversal command-line tool and (on Linux) puts it in the ~/.local/bin/ directory by default. Installation on Windows, even though not tested, should be also possible.

Data format

The tool works with the .cupt data format.

Configuration

To train an MWE identification model, you will need to specify the configuration of the system first. This is done using the Dhall programming language. Configuration determines, in particular, the feature templates to be used for MWE identification. The default configuration can be found in the config/config.dhall file.

Training

Assuming that you have:

• train.cupt: training file with MWE annotations in the .cupt format,
• dev.cupt: development file with MWE annotation in the .cupt format (optional),
• config/config.dhall: configuration file with feature templates,

and that you wish to identify MWEs of the category VID (each MWE in the .cupt file should be marked with its category), then you can use the following command to train the corresponding model and store it in the VID.model file:

traversal train -c config/config.dhall -t train.cupt -d dev.cupt --mwe VID -m VID.model

MWE identification

Once you train the model, you can perform MWE identification using the following command:

traversal tag -c config/config.dhall --mwe VID -m VID.model < test.cupt

where test.cupt is the input file (in the .cupt format). The command outputs the MWE identification results to stdout. You must use the same configuration (-c config/config.dhall) as during training.

MWE annotations already present on input (if any) will be copied on output. This allows to annotate a single file based on several models corresponding to different MWE categories.

MWE segmentation

By default, all adjacent dependency nodes marked as MWEs of the same category are assumed to form a single MWE occurrence. Another segmentation heuristic available in the system is to split each group of adjacent MWE-marked nodes into two (or more) distinct MWEs, depending on how many nodes with a certain POS value it contains.

For instance, to divide each MWE-marked group into two (or more) distinct MWEs when it contains two (or more) verbs, use the following command:

traversal tag -c config/config.dhall --mwe VID -m VID.model --split VERB < test.cupt

Both heuristics provided in the system are rather rudimentary and we plan to replace them by a more advanced solution to MWE segmentation in the future.

Clearing

If you want to remove MWE annotations from a given test.cupt file, use:

traversal clear < test.cupt